Frost damage in early spring and low temperature freezing injury in winter bring about serious losses to agricultural and forestry plants. Frost damage and freezing injury occurs with certain characteristics. The extent of freezing injury depends on low temperature climatic conditions and biological sensitivity of resistance to freezing. Firstly, frost damage usually occurs under thermal inversion conditions during early spring. Plants and the ground absorb large amounts of solar radiation in the daytime, and consecutively radiate heat away from the surface to the atmosphere in the nighttime, so that air temperature near ground falls quickly, resulting in thermal inversion. Secondly, plants are most vulnerable to frost and freezing during the period of frequent late frost (which is also the period of bud burst and leaf spreading). By making full use of distribution characteristics of temperature field under thermal inversion, temperature of this lower layer can be effectively increased to avoid and reduce frost damage, and also advance maturing stage of plant products after convecting thermal inversion layer near ground by wind machines to push upper warmer air downward to plant canopy below. However, determination of the start and stop timing of wind machines remains a key and difficult issue of this domain.
Previous research results show that existing wind machine systems used for plant frost protection only rely on a critical damage temperature, wind speed or an empirical temperature as control condition to start wind machines. For example, Snyder (2005) reported that the startup temperature of frost protection method used for orchards and vineyards in Europe and America is less than 0° C. and wind speed is used as the condition for the start of wind machines: when wind speed exceeds 2.5 m/s, wind machines will be stopped. As for anti-frost fans used in Japan for tea plants, temperature of canopy or a lower place in tea field is normally used as a condition to start wind machines. Empirical value of this temperature is about 3° C. Ribeiro (2006) evaluated temperature rise effect after operation of wind machines using the principle of energy balance and he figured out that the temperature at 1.5 m above ground could be chosen for the start of wind machine. USA patents numbered U.S. Pat. No. 4,753,034, U.S. Pat. No. 5,244,346 and U.S. Pat. No. 4,501,089 disclose devices that use centrifugal wind machines to suck lower cold air up to the sky, or use axial flow wind machines to blow upper warm air downward, to prevent frost for fruit trees and grape vines, but corresponding control method and apparatus were not discussed. Japanese patents numbered JP2007000096, JP2000050749 and JP10113076 disclose axial flow wind machines used for tea farms and orchards, but control method and apparatus of the wind machines were not discussed. In China, Hu Yongguang et al. (2007) developed a frost protection system with an axial flow wind machine as its core part and tested wind speed distribution, temperature distribution and temperature rise effect, but control method and apparatus for wind machines were not involved. Li Pingping et al (2008) reported spatiotemporal distribution characteristics of near-ground temperature of a tea farm under thermal inversion conditions in early spring. However, it only provided full demonstration for feasibility of application of elevated frost protection wind machines based on air turbulence method.
In fact, if a temperature or wind speed is used as the condition for system startup, the system will push upper cold air downward to the plants in case of absence of thermal inversion, so that frost damage would be aggravated (incorrect operation), resulting in greater loss. On the other hand, the frost protection effect of wind machine will be less (null operation), and consequently energy will be wasted if the system starts with less thermal inversion temperature difference (difference between temperature above near-ground thermal inversion layer and air temperature at plant canopy).